on 10-06-2021 12:26 PM
No domain is an indication that the email address has been entered incorrectly or the email address is not active on the TalkTalk Mail platform.
When was the last time you signed in to the email account? If more than 180 days then the mailbox will be flagged for deletion for non use. Deleted Email error message
Are you a TalkTalk Consumer customer and if so is the email address listed in your customer MyAccount? Managing your email in My Account
on 10-06-2021 10:45 AM
The advice is in this help page Changing your email password and if you've forgotten the password just use the Forgotten password button.
The password reset does rely on having previously set up password recovery and keeping the alternate email address and mobile number updated.
If there are no alternate reset paths available or you don't recognise them from the hints given then let us know.